Start by checking to make sure you have the correct size bulbs
installed. If they are correct then replace the flasher relay. It is
located Front seating area, driver side, under dash, mounted on top of
brake pedal. If that does not cure the fast flashing, you may have a
bad ground connection or the turn signal combination switch may be bad.

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You 2004 Ford F150 use OEM 3157 bulbs.Inside these bulbs,there are TOTAL of TWO bulb filaments.When one of the two burned out,it will cause turn signal problem.Stock up with 4 bulbs before you start.Auto part store sell then is pair. I recommend you replace then in part and save the good on for emergency.I attached a photo of the 3157 bulb below.As you can see there are two filaments inside the bulb.Check the bulb one by one and you will get the signal system working.====There are protective grease inside the base of the socket.Make sure it's not blocking the four contact points (both side of the socket).====

It is not the flasher if the hazards work, it is 1 flasher for both turn signals and the hazard lamps. It is in a relay block ( along with the blower motor relay ) under the dash, about in line with the steering wheel column ).

Try tilting the steering wheel all the way up, and test again.

The other quick test is to cycle the hazard switch a few times, to see if it cleans the contacts for the turn signal on the MFS,

I have a 2001 Ford F150, my brake lights dont work. Ive replaced the bulbs, checked the fuses,and the brake switch on side of the brake pedal was replaced. I still dont have any brake lights. My third brake light works fine, along with my running lights,flashers, blinkers.